Add prometheus metrics endpoint

[taonic]

Key changes:

Add Prometheus metrics endpoint, e.g. :9090/metrics
Add key metrics to codec and crypto package. See metrics/constants.go
Add sample grafana dashboard: dashboards/grafana-dashboard.json

Todo:

Add metrics to proxy methods.

Test 1:

encryption:
  caching:
    max_cache: 100
    max_age: "10m"
    max_usage: 100

Test 2:

encryption:
  caching:
    max_cache: 1000
    max_age: "10m"
    max_usage: 1000